1. Field of the Invention
The present invention relates to a wafer holder of a FIB (Focused Ion Beam) apparatus. More particularly, the present invention relates to a wafer fixing unit with vertically projecting parts capable of maintaining the flat zone of a wafer in accurate alignment.
2. Description of the Related Art
In a FIB (Focused Ion Beam) apparatus, a defective portion of a wafer is milled by an accelerated ion beam (e.g., a gallium positive ion beam) directed onto the defective portion. An image is formed from secondary by-products (e.g., secondary electrons, secondary ions, neutrons and so on) emitted from the surface of the defective portion. Such an FIB apparatus is used for analyzing defective portions of layers formed on the wafer as well as a defective portion of the wafer itself. (The defective portions of layers and the defective portion of the wafer are hereinafter referred to as the "sample").
The FIB apparatus operates on portions of the wafer identified by a defect detecting apparatus, such as that manufactured by KLA Corporation. The positions of defects on the wafer are determined by coordinates relative to an origin (0,0) set at a lock point of the defect detection apparatus. A coordinate calibration step is necessary so that FIB coordinates relative to an FIB lock point correspond to the coordinates relative to the lock point of the defect detection apparatus. For an accurate relationship between the coordinates of the FIB lock point and the lock point of the defect detection apparatus, the flat zone of the wafer must be aligned in flush contact with a wafer fixing pin of a wafer fixing unit included in the FIB apparatus.
The structure of a conventional wafer fixing unit and a method for aligning the wafer using the unit are briefly described next.
FIG. 1 is a schematic plan view of a conventional wafer fixing unit of an FIB apparatus, for example, the FIB apparatus manufactured by the MICRION company. As oriented in FIG. 1 and described hereafter, the unit is in the horizontal plane, and the top of FIG. 1 corresponds to a rear area of the unit, while the bottom of FIG. 1 corresponds to a forward area of the unit.
The conventional wafer fixing unit of the FIB apparatus includes a wafer fixing pin 14 having a first plate-shaped support member 10, and a second substantially T-shaped support member 12 that has a narrow base end toward the rear and a wide end toward the forward area. The first support 10 has a semicircular depression with a predetermined depth in a hinge area D into which is fitted the narrow base of the second support 12. A pair of plate-shaped side supports 16, for supporting the wafer fixing pin 14, are fixed on the left and the right of the first support 10 by means of clamp screws A'. A forward plate-shaped support 18 is arranged forward of the first support 10 between the pair of side supports 16. A pair of springs 20 connect the first support 10 to the forward support 18 on either side of the second T-shaped support 12. To the rear of the first support 10 and the pair of side supports 16, a rear support 22 is arranged for supporting the first support and side supports.
The view defined by the section marked II-II' is illustrated in FIG. 2. Referring to FIG. 2, the rear area of the unit is toward the left, and a single projecting part 13 rises vertically, i.e., perpendicularly to the T shaped support 12, at a predetermined height. The vertical surface 13a of the projecting part 13 facing the rear represents the perpendicular surface that contacts the wafer. Referring to FIG. 1, the projecting part 13 (FIG. 2) is formed on the second support 12 so that the rear-facing contact surface 13a falls along the line I-I' where the narrow part of the T-shaped support 12 meets the wide part of the T-shaped support 12. As drawn, the wafer fixing pin 14 is in a rest position when the first support 10 toward the rear of the unit is adjacent to the rear support 22, and the contact surface of the projection part 13 (FIG. 2) is on the line I-I'. When the wafer is aligned using this conventional wafer fixing unit, the flat zone of the horizontal wafer is in contact with the rear surface 13a of the projecting part 13 in FIG. 2, and the flat zone edge is oriented in a direction that is substantially left to right.
The second support 12 is hinged to the first support 10 in hinge area D by means of a clamp screw A so that the wide end of the second support 12 can be rotated left and right through a predetermined range. A pair of grooves B are formed at predetermined positions on the left and right sides, respectively, along the narrow part of the second support 12. At predetermined positions of the forward support 18 on the left and the right of the second support 12, a pair of projecting pegs C are formed, respectively. The pegs are slightly smaller in diameter than the grooves B so that the pegs C can engage the grooves B when the second support 12 is moved forward and rotated left or right. Therefore the pair of projecting parts C and the pair of grooves B are used for locking the wafer fixing pin 14 in a predetermined open position.
Generally, the width (measured left to right) of the wide end of the second support 12, is approximately 19 mm. The width of the contact surface of the projecting part 13 is approximately 6.5 mm.
FIG. 3 is a flow chart showing the four steps of the wafer alignment process using the conventional wafer fixing unit. The wafer fixing pin 14 begins in the rest position. At step S24, the pin 14 is moved forward by an external force. At step S26, either of the grooves B is selected, and the selected groove B is engaged with the corresponding projecting peg C on the forward support 18 by rotating the second member 12 slightly. This locks the wafer fixing pin 14 in an open position, i.e., it fixes the pin. It is possible to fix the wafer fixing pin in an open position because the second support 12 of the pin 14 is hinged in the area D.
At step S28, a wafer is loaded onto the wafer fixing unit. The left to right position of the wafer on the wafer fixing unit is determined by wafer-supporting screws (not shown) located to the rear of the wafer fixing unit. The round edge of the wafer, opposite the flat zone, is supported by the wafer-supporting screws. At this time, the edge of the flat zone of the wafer is oriented along the line III-III' (FIG. 1), which is slightly forward of the line I-I' where the contact surfaces are positioned in the rest position. At step S30, the wafer fixing pin 14 is released from the projecting peg C, and it is moved rearward by the elasticity of the pair of springs 20, pushing the wafer at the same time until the wafer alignment is completed.
However, the wafer alignment using the conventional wafer fixing unit suffers some drawbacks. Referring to FIG. 4, during the fourth step S30, while the flat zone of the wafer 15 is pushed by the projecting part 13 of the second support 12 during its movement rearward, the wafer may be rotated by some amount and the final alignment of the wafer flat zone 15 might be slanted. As a result, the flat zone of the wafer does not come in flush contact with the projecting part 13 of the second support 12 and the alignment of the wafer is not accurate.
In the event that the flat zone 15 of the wafer is not in flush contact with the projecting part 13 of the wafer fixing pin 14, i.e., the wafer is not accurately aligned, and the lock point of the FIB is not at the designated origin (0,0) of the FIB coordinate system. Therefore the coordinate calibration of the FIB apparatus with respect to the defect detection apparatus, such as the KLA, is not accurate. As a result, the actual positions of defects cannot be accurately found in the FIB using the coordinates of the defects from the KLA. This makes the sample defect analysis difficult or impossible.